The present invention relates generally to wind turbines, and in particular to techniques for stabilizing towers of wind turbines in both new and retrofit applications.
Wind turbines are generally regarded as an environmentally safe and a desirable source of renewable energy. In summary, a wind turbine harnesses the kinetic energy of wind and transforms this kinetic energy into electrical energy. Thus, electrical power can be generated with virtually zero emissions, unlike existing natural gas-fired or coal-fired power generation technologies. To maximize the efficacy of power generation and to simplify connection to a power grid, several wind turbines are often located in proximity to one another in what are generally referred to in the pertinent art as “wind farms.” Advantageously, these wind farms are located in regions having relatively strong winds.
Many wind turbines are rapidly approaching the end of their design life. With new wind technology development in recent years, upgrading or replacing old wind turbines with newer ones can potentially raise yield by a factor of two or more. Currently “repowering” old wind fleets is a vital and growing business among major turbine manufacturers. These new technologies include the use of larger turbines or the location of hubs at higher positions, if wind conditions allow, thus enabling an increase in annual energy production (AEP).
The overall structural design of the tower and foundation structure of a wind turbine is determined by the size of the turbine, the dynamic wind loads wider various turbine operational modes and by extreme loads imposed during high wind speed conditions (during which the turbine may or may not be operational). To minimize fatigue loading, the stiffness of the tower and foundation structure should be desirably sized such that the overall natural frequency of the wind turbine/foundation system is outside the frequency range of excitation due to the rotor operation and the dynamic wind loading. A typical industry practice is to size the stiffness of the tower and foundation structure such that the overall system natural frequency is higher than the excitation from the rotor revolution but lower than the excitation from the blade passing the wind turbine tower. At greater heights (>120 m) found in advanced wind technologies, standard tubular towers are not cost effective. Even if larger towers were to become available, the feasibility of such is limited due to the limited availability and increasing expense of larger foundation structures. The tower and foundation thus becomes a limiting factor in upgrading to a larger turbine. Replacing the entire tower and foundation structure requires significant cost and time. Accordingly, advanced tower designs for both new and retrofit applications are necessary to advance wind technology and reduce cost.
Guy wire or cable stabilized towers have long been a proven technology for static applications (antenna masts, bridges, etc.). Traditionally, the cables were used for small wind turbines. Only recently have they found their way into the large utility wind turbine market. By attaching cables to the tower structure, the additional bending moment from a larger turbine is offset by the cable tension forces. From an attachment point of the cable and downward, the bending moment is significantly reduced. The moment acting on the foundation structure is also reduced. Connector technologies link the guy wire to the tower and transfer operating loads efficiently from the tower to the cables. Existing designs use cable and cable connectors which are either welded or bolted to the outside or inside of the tower shell/flange. In other designs, connectors fastened to the inside of the tower shell require the cable to penetrate the shell. This design creates stress concentrations around the perimeter of the opening, increasing complexity of the design. Welded and bolted connections are typically more difficult to design for fatigue life.
Therefore, there is a need to design a system and method for stabilizing a wind turbine that will provide the necessary offset for increased bending loads for larger turbines, at increased tower heights and higher winds. There is also a need for a more robust design of a connector that is not only cost effective, but has an increased fatigue life, and provides for ease in manufacture and assembly.